Projectors for projecting images on the basis of image signals have been used for presentation in the meeting or home theaters. As a light source for the projector, a light source capable of rapidly turning on and stably ensuring the quantity of light with high brightness is used to rapidly obtain a bright projected image, or a light source capable of adjusting the quantity of light without changing an emission spectrum is used to obtain a bright projected image most suitable for a projection environment.
Discharge lamps, such as a halogen lamp, a metal halide lamp, and a high-pressure mercury lamp, have come into widespread use in the projectors on the market. This is because the discharge lamp can obtain high brightness.
The discharge lamp needs discharge electrodes for making a discharge medium, such as gas filled in the lamp, emit light by discharge. However, the discharge lamp has the following problems due to the discharge electrodes.
The abrasion of the discharge electrodes due to discharge causes the distance between the electrodes to increase, which results in a variation in the emission spectrum. In addition, a light source becomes large, which may have an effect on a projected image. Further, the orbit of discharge arc moves, which may cause a flicker due to an arc shift. Further, in the lost phase in which the electrodes are abraded to a large degree, there has a problem in that discharge may not occur.
Further, the internal temperature and pressure of the discharge lamp need to increase and the discharge medium, such as gas, needs to be sufficiently excited until the quantity of light emitted from the discharge lamp reaches a predetermined value. In this case, it takes a predetermined amount of time to obtain the necessary amount of light. In addition, a current equal to or larger than a predetermined value is needed to maintain the discharge after the discharge lamp is turned on. In this case, it is difficult to adjust the quantity of light. Therefore, a projector having an iris for adjusting the quantity of light around a light source has been proposed.
In order to solve these problems due to the electrodes, JP-A-2001-155882 discloses a projector using an electrodeless lamp as a light source device. In the light source device of the projector, a magnetron, which is a kind of vacuum tube having electrodes, generates microwaves, and the microwaves are radiated onto an electrodeless lamp having rare gas or rare-earth metal halogen compound, serving as a discharge medium, filled therein to emit light by plasma discharge. This structure makes it possible to provide a projector having an electrodeless lamp, which is a point light source having high brightness and a long life span, as a light source.
In order to start the light emission of the electrodeless lamp, a microwave having predetermined power needs to be radiated at once to start plasma discharge.
It is necessary to preheat the magnetron for a predetermined period of time in order to start the magnetron to obtain a predetermined microwave. For example, JP-A-9-82112 discloses a power supply for an electrodeless lamp. In JP-A-9-82112, when the preheating temperature is high, the frequency characteristics of microwaves generated by the magnetron deteriorate. Therefore, temperature control is performed to reduce the preheating temperature when the electrodeless lamp is turned on.
FIG. 13 is a diagram illustrating the frequency characteristics of the microwaves generated by the magnetron. The frequencies of the microwaves are distributed with the center at a frequency of about 2.45 GHz, and many noise components are included in a frequency band of about 2.25 to 2.65 GHz. It has been known that, when output microwave power varies, the frequency characteristics vary.
However, in the light source devices disclosed in JP-A-2001-155882 and JP-A-9-82112, the magnetron requiring preheating is used for a source for generating microwaves. Therefore, it takes a predetermined amount of time for the electrodeless lamp to start emitting light, which makes it difficult to rapidly turn on the electrodeless lamp.
Further, in order to start the light emission of the electrodeless lamp, a microwave having predetermined power needs to be radiated at once to start plasma discharge. However, this is not disclosed in JP-A-2001-155882 and JP-A-9-82112, and thus it is difficult for the related art to reliably turn on the electrodeless lamp.
Furthermore, when output microwave power varies, the frequency characteristics of the magnetron vary, which makes it difficult to adjust the quantity of light without changing the emission spectrum of the electrodeless lamp.
As described above, the light source devices of the projectors according to the related art have problems in that it is difficult to rapidly and reliably turn on the light source device and the light source device does not stably obtain a predetermined amount of light.
In order to solve the above-mentioned problems, it is an object of the invention to provide a projector having a light source device capable of being rapidly and reliably turned on and stably obtaining a predetermined quantity of light and a method of turning on the light source device of the projector reliably and rapidly.